The dimming methods for the conventional cold cathode fluorescent lamp (CCFL) generally include duty cycle control, frequency control and voltage control.
R.O.C. Patent Publication No. 504101 discloses an “Actuation device for high luminous fluorescent lamps”. In mainly includes a high frequency oscillator, a pulse width modulator, a first and a second power switches, and a piezoelectric transformer. The high frequency oscillator generates a high frequency AC signal which is transformed to a PWM resonant frequency signal through the pulse width modulator. The positive half cycle and the negative half cycle of the PWM resonant frequency signal drive respectively the two power switches, and then they actuate respectively two input ends of the primary coil of the piezoelectric transformer. Thereby the piezoelectric transformer generates a pulse wave to actuate the CCFL. The CCFL actuation device thus formed has higher transformation efficiency and a step-up ratio of high pulse wave. In short, that patent adopts the pulse width modulation to actuate the piezoelectric transformer through a constant high frequency signal, and control the power switches through a non-continuous conductive mode of constant conduction. This method generally controls the average current of the fluorescent lamp through a high frequency (>100 Hz) not visible to human eyes and by modulating the OFF period of the duty ratio. The frequency is constant.
Another control method being taken seriously is digital dimming or called burst dimming. This method aims to control the lamp current to function at a steady nominal value. Then a low frequency dimming (LFD) control voltage is used to regulate the duty/close cycles (i.e. ON/OFF cycles) of the lamp. The average illumination of the lamp is direct proportional to the duty cycle. Hence the average illumination can be controlled. However, the conventional techniques mostly adopt hard-start at a constant frequency. Referring to FIGS. 1 and 2, the general start voltage is about 2–4 times of the duty voltage. In the OFF cycle, the OFF potential is zero (namely no current). For a gas discharge lamp, a certain amount of electric energy is required to actuate the ignition. But such a control method is set in a condition of a lower dimming value (usually below 30%). As a result, the electric energy required by the gas discharge lamp in the duty cycle often is not adequate and results in partial ignition of the gas discharge lamp. When the gas discharge lamp is used as the light source of a display screen, the illumination is not even or flicker occurs. Hence the present fabrication technique can only set the dimming value of the gas discharge lamp in the range between 30% and 100%. If the dimming value is lower than the range mentioned above, the gas discharge lamp is OFF and not operable. In some special environments where the display illumination is low (such as the vehicles moving in a bright sunshine environment), the control method previously discussed is useless. To overcome this problem, some adopt the approach of neglecting the shortcomings of uneven illumination or flicker phenomenon. But due to the actuation electric energy is not adequate, to restore the dimming cycle in the standard dimming range the lamp has to be turned ON and OFF repeatedly. Namely, a start cycle must be generated first when to restore the dimming value to a higher setting condition. Each start cycle will generate a higher start voltage and a surge current. This is harmful to the electrodes of the gas discharge lamp. Hence the conventional digital dimming method has a negative impact to the service life of the lamp.